1. Field of the Invention
The present invention relates to a negative polymerizable composition and a negative planographic printing plate precursor, and in particular to a negative polymerizable composition and a negative planographic printing plate precursor that allow highly sensitive writing by infrared laser.
2. Description of the Related Art
PS plates having an lipophilic photosensitive resin layer formed on a hydrophilic support, which have hitherto been widely used as planographic printing plate precursors, have been produced commonly by mask exposure (surface exposure) through a lith film and by dissolving and thus removing the nonimage regions. In recent years, digital technology, in which image information is processed, stored, and outputted electronically by computer, is becoming more and more popular. Accordingly, many newer image-output methods compatible with the digital technology have been commercialized in a greater number. As a result, there is an urgent need for the “computer to plate (CTP) technology” that allows direct production of printing plates by scanning a high-directivity light such as laser beam according to digitalized image information without using the lith film, and thus for the planographic printing plate precursors that are compatible with the CTP technology.
As the planographic printing plate precursor compatible with such scanning exposure, a planographic printing plate precursor wherein an lipophilic photosensitive resin layer (hereinafter, referred to as a recording layer) containing a photosensitive compound that can generate an active species such as free radical, Bronsted acid, or the like by laser exposure is formed on a hydrophilic support was proposed and already commercialized. It is possible to obtain negative planographic printing plates by scanning the planographic printing plate precursor with laser according to digital information, generating such active species and thus causing physical and chemical changes on the photosensitive layer, insolubilizing the exposed regions, and developing the images thereon. In particular, a planographic printing plate precursor wherein a photopolymerizable recording layer containing a photopolymerization initiator superior in sensitization speed, an addition-polymerizable ethylenically unsaturated compound, and a binder polymer soluble in alkaline developing solution, and additionally an oxygen-blocking protective layer as needed are formed on a hydrophilic support is a possible candidate for the desirable printing plate precursor superior in printing properties, as it is superior in productivity and easier in developing processing and provides a printing plate precursor superior in resolution and inking property.
Normally, negative-type image-recording materials such as the planographic printing plate precursors described above employ a recording method of forming image regions by causing a polymerization reaction by the radicals generated by light or heat as initiators and hardening the recording layer in exposed regions. Such negative-type image-forming materials, which are weaker in image-forming property than positive-type image forming materials in which the recording layers are solubilized by the energy by infrared laser irradiation and form firm image regions by accelerating hardening by polymerization, are generally heat-treated prior to processing in the developing process. The negative-type image recording materials demanding such a post-heating treatment are, for example, the recording materials consisting of resol and novolak resins described in U.S. Pat. No. 5,340,699 and the like. In particular, when an aluminum support is used, there was the problem that it was difficult to obtain sufficient sensitivity because the energy by infrared laser irradiation could not be used for initiation and propagation of the polymerization reaction for image formation due to the diffusion thereof to the support higher in heat conductivity.
The combinations of an initiator and a dye having high photosensitivity in the infrared region that could solve the problems above are, for example, the borate/cyanine colorants described in U.S. Pat. No. 5,496,903, halogenated s-triazine/cyanine colorants, and the like, but they still carried the problem of extremely lower storage stability. For example, the initiator system containing a polycarboxylic acid described in Japanese Patent Application National Publication (Laid-Open) No. 2002-537419 was known as an initiator system that was improved in storage stability, but it still does not have sufficient storage stability.
In addition, particular initiators absorbing light in the infrared region are described in Japanese Patent Application Laid-Open (JP-A) Nos. 63-142346 and 9-100306, but there was a problem that the particular initiator described therein, which are highly hydrophilic, did not provide sufficient sensitivity when an oxygen-blocking layer is present as the upper layer.